The camptothecin analog SN-38 (7-ethyl-10-hydroxycamptothecin) is an active metabolite of the antitumor drug irinotecan. It is ˜1000× more active than irinotecan, yet has not been therapeutically useful due to extremely poor aqueous solubility (17 uM) and rapid clearance.

Irinotecan itself is used clinically and has shown activity in leukemia, lymphoma, colorectal, lung, ovarian, cervical, pancreatic, stomach, and breast cancers. Several studies have indicated that the camptothecins in general owe their antitumor effects to inhibition of topoisomerase I, and that efficacy is related to maintaining inhibition of this enzyme for prolonged periods (“time over target”). In order to maintain efficacious levels of camptothecins for a sufficient time, it is typically required that fairly high doses of drug be administered to counter the relatively rapid clearance rate of the drug from the system. This results in high maximal drug concentrations (Cmax) at early times after administration, which is thought to result in toxicities such as the life-threatening diarrhea that is the dose-limiting toxicity for irinotecan. Given the high potency of SN-38, it would be desirable to provide the drug as a prolonged infusion at a steady-state concentration sufficient for inhibition of topoisomerase I but lower than the toxic concentration. Clinical trials using prolonged infusion of irinotecan via pumps have corroborated this hypothesis, yet this is not a feasible therapeutic strategy for SN-38 due to poor solubility in dosing formulations.
Irinotecan is converted into SN-38 by hepatic carboxyesterase, and then metabolized by hepatic UGT1A to its 10-glucuronide, SN-38G. Glucuronidation facilitates biliary excretion and intestinal bacterial glucuronidase causes reconversion of SN-38G to SN-38. Unless intestinal UGT1A converts the drug back to the inert SN-38G, the SN-38 can cause toxic effects on the intestine. Thus, SN-38G may act both as the source of the toxic SN-38 and a protection against the severe diarrhea caused by SN-38. In general, high levels of SN-38 result in increased glucuronidation to SN-38G, increased excretion of SN-38G into the intestine, and bacterial deglucuronidation that results in gastrointestinal toxicity.
Slow release of SN-38 from a soluble, long-lived circulating conjugate rather than from a prodrug such as irinotecan would seem to provide a solution to these problems, and various conjugation strategies have been applied to SN-38. Conjugation to poly(ethylene glycol) (PEG) via an oxygen-20 glycinate ester (U.S. Pat. No. 8,299,089) provides a relatively aqueous-soluble conjugate that releases free SN-38 relatively rapidly (t1/2=12 h) due to ester hydrolysis. Another ester-linking chemistry to a polyol polymer with fast release of SN-38 has been disclosed (US Patent Publication 2010/0305149 A1). Conjugation to a polyglutamate-PEG block copolymer via ester linkages to oxygen-10 provides a micellar conjugate that again releases free SN-38 by ester hydrolysis (PCT Publication WO2004/039869). Due to the instability of esters in aqueous media, possibly accelerated by esterases in plasma, such ester-based conjugation strategies for SN-38 are not appropriate to support low-dose, long-term exposure to SN-38, and levels of SN-38 typically fall below the efficacious level between dosings. The conjugates are typically administered at high levels, giving high maximal concentrations of SN-38 and resulting in formation of high levels of SN-38G.
PEG-SN-38 conjugates having more controlled release rates have been disclosed in PCT Publication WO2011/140393. These conjugates release SN-38 through a beta-elimination mechanism, with rates controllable over a wide range through selection of the appropriate linker. Coupling a macromolecule to this drug was through a condensation product of azido with a cyclic alkyne resulting in a relatively insoluble conjugate. The subgenus claimed herein has improved solubility due to the presence of a simple amide linkage, and is stable in vitro in buffer at room temperature. We have now unexpectedly found that through appropriate selection of the release rate the in vivo formation of SN-38G may be reduced while simultaneously providing a long-term exposure to active SN-38. The present invention provides conjugates that are designed to release free SN-38 through a non-enzymatic beta-elimination mechanism at rates that enable low-dose, long-term exposure regimens for SN-38 and which further reduce the amount of SN-38G formed during administration.